Conventionally, in a Scott-connected transformer or the like, for example, a coupling coil has been employed for the following reason: That is, a Scott-connected transformer 10 shown, for example, in FIG. 5 includes an iron core 11, a main-phase primary coil 12, a teaser primary coil 13, a main-phase secondary coil 14, and a teaser secondary coil 15. Each of the coils 12, 13, 14, and 15 is configured such that a conductor wire is wound around the iron core 11. One end of the teaser primary coil 13 intersects and is connected to the main-phase primary coil 12 at a middle point N thereof, which is a midway portion thereof. A three-phase power supply that is not shown is connected to a terminal V of the teaser primary coil 13 and terminals U and W of the main-phase primary coil 12.
A first single-phase load 91 is connected to terminals 1u and 1v of the main-phase secondary coil 14, which is one of the secondary coils 14 and 15. A second single-phase load 92 is connected to terminals 2u and 2v of the teaser secondary coil 15. The voltage outputted from the main-phase secondary coil 14 and the voltage outputted from the teaser secondary coil 15 are shifted from each other by a phase difference of 90°. In this case, mutual induction occurs between the main-phase primary coil 12 and the main-phase secondary coil 14 and between the teaser primary coil 13 and the teaser secondary coil 15.
FIG. 6 shows current flowing through the Scott-connected transformer 10 in FIG. 5 in a state in which only the first single-phase load 91 is connected to the terminals 1u and 1v of the main-phase secondary coil 14 but the second single-phase load 92 is not connected to the terminals 2u and 2v of the teaser secondary coil 15. Current i1m flowing through the main-phase primary coil 12 and current i2m flowing through the main-phase secondary coil 14 flow in such a way that the ampere-turns of the two coils 12 and 14 cancel each other out. In this case, the short-circuit impedance in the main-phase primary coil 12 and the main-phase secondary coil 14 is the leakage impedance between the two coils 12 and 14.
In contrast, FIG. 7 shows current flowing through the Scott-connected transformer 10 in FIG. 5 in a state in which only the second single-phase load 92 is connected to the terminals 2u and 2v of the teaser secondary coil 15 but the first single-phase load 91 is not connected to the terminals 1u and 1v of the main-phase secondary coil 14. Current i1t flowing through the teaser primary coil 13 flows so as to cancel the ampere-turns of current i2t flowing through the teaser secondary coil 15 and then splits at the middle point N into current i1t1 and current i1t2, which flow through the main-phase primary coil 12.
In this case, the short-circuit impedance on the teaser side is the sum of the leakage impedance between the teaser primary coil 13 and the teaser secondary coil 15 and the leakage impedance between a U-side main-phase primary coil 121 and a W-side main-phase primary coil 122. Therefore, to reduce the short-circuit impedance on the teaser side, it is necessary to reduce the leakage impedance between the U-side main-phase primary coil 121 and the W-side main-phase primary coil 122.
In the configuration described above, employing the structure of a coupling coil as the structure of the main-phase secondary coil 14, as shown in FIGS. 8 and 9, allows reduction in the leakage impedance between the U-side main-phase primary coil 121 and the W-side main-phase primary coil 122. A coupling coil refers to a structure having a function of improving magnetic coupling between a plurality of windings set apart from each other.
The structure of the coupling coil is configured, for example, as follows: That is, the main-phase secondary coil 14 is divided at a middle portion into two coils, a U-side main-phase secondary coil 141 and a W-side main-phase secondary coil 142. The U-side main-phase secondary coil 141 and the W-side main-phase secondary coil 142 are connected in parallel to each other. The U-side main-phase secondary coil 141 faces the U-side main-phase primary coil 121, and the W-side main-phase secondary coil 142 faces the W-side main-phase primary coil 122.
In this configuration, the second single-phase load 92 is connected to the terminals 2u and 2v of the teaser secondary coil 15, and the current i1t having flowed through the teaser primary coil 13 splits into current flowing through the U-side main-phase primary coil 121 and current flowing through the W-side main-phase primary coil 122, as shown in FIG. 8. As a result, mutual induction between the main-phase primary coil 12 and the main-phase secondary coil 14 induces electromotive force in the main-phase secondary coil 14. Current i2t1 therefore flows through the U-side main-phase secondary coil 141 so as to cancel the ampere-turns of the current lit′ flowing through the U-side main-phase primary coil 121. Similarly, current i2t2 flows through the W-side main-phase secondary coil 142 so as to cancel the ampere-turns of the current i1t2 flowing through the W-side main-phase primary coil 122.
The current i2t1 and the current i2t2 circulate through the path formed of the U-side main-phase secondary coil 141 and the W-side main-phase secondary coil 142. The circulating current i2t1 and current i2t2 cancel the ampere-turns of the current flowing through the U-side main-phase primary coil 121 and the current flowing through the W-side main-phase primary coil 122, into which the current i1t flowing through the teaser primary coil 13 splits. As a result, the magnetic coupling between the U-side main-phase primary coil 121 and the W-side main-phase primary coil 122 is improved, whereby the leakage impedance between the U-side main-phase primary coil 121 and the W-side main-phase primary coil 122 can be reduced.